Most automotive vehicles are powered by internal combustion engines comprising several complementary cylinders in which closely fitting reciprocating pistons accommodate the introduction of a fuel-air mixture, the compression of the mixture, the combustion of the mixture and the exhaust of combustion products. The fuel may be gasoline, diesel fuel, alcohol-fuel mixtures, alcohol, bio-fuel compositions, or other carbon-containing compositions that are combustible with air. Lubrication of moving parts is accomplished with a suitable hydrocarbon-containing liquid that is pumped from a crankcase at the lower part of the engine and distributed over moving parts of the engine. Circulation of the lubricating oil is completed as it drains back into the sump. The volume of the oil may be several quarts depending on the size of the engine.
At the conclusion of the combustion processes occurring in the cylinder, the cylinder will contain a variety of chemical species including water vapor, unburned fuel and particulates. The majority of these will be exhausted from the cylinder during the exhaust stroke but some fraction will be conveyed past the piston rings into the crankcase (sometimes called blow-by gases) and into contact with the lubricating oil. Thus, as the engine is operated and the oil is circulated it accumulates particulate matter which may be removed by circulating the oil through a filter. But the oil also accumulates water, un-burned fuel material, and other liquid or gaseous combustion by-products which cannot be removed through conventional filtration techniques.
At normal engine operating temperatures these combustion by-products are often sufficiently volatile that they may be ejected from the oil by evaporation. However when an engine is regularly operated for only relatively short periods, particularly at low ambient temperatures, the oil temperature does not increase sufficiently to vaporize and expel these contaminants and they progressively accumulate. This accumulation of water, fuel, and combustion by-products, principally liquids, dilute and contaminate the oil such that it may have to be drained from the engine before its useful properties would have otherwise been depleted.